In a conventional steering system such as for outboard motors used on boats, the propulsion unit or engine typically mounted on the transom of the boat is pivoted about a vertical steering axis upon steering actuation by the operator or driver at the helm. The propulsion unit comprises a powerhead for housing an engine from which extends a vertically disposed drive shaft having a propeller, including a rudder, mounted at the end. One typical steering system for a boat having a transom mounted engine comprises a steering cable, such as a push-pull cable, extending between the steering helm and the propulsion unit so that steering at the helm actuates the cable for causing steering movement of the propulsion unit about a steering axis. Hydraulic activated steering means can be used in place of the cable steering, wherein hydraulic fluid, e.g. oil, is pumped from the steering helm through conduits or lines (typically a plastic line) to a cylinder-piston control means in response to rotation of the steering wheel in one direction or the other. Actuation of the cylinder-piston control means (commonly referred to as the steering cylinder, steering module, or helm pump) actuates the steering mechanism of the propulsion unit, thereby turning the propulsion unit in a common direction.
Generally, the steering cylinder to effect pivoting of the propulsion unit relative to the vertical steering axis includes a hydraulic cylinder having opposed end caps and mounted longitudinally on a horizontally disposed support rod which is fixed against axial movement by suitable bracketry (but may be tiltable about a horizontal axis). The steering axis is normally envisioned as being perpendicular to the surface of the water. The support rod is provided with a piston which is fixed centrally in the cylinder, and hydraulic conduits or lines opening to the cylinder are spaced on each side of the piston. The hydraulic cylinder is moveable relative to the piston and to the support rod in response to selective application thereto of hydraulic fluid from the operator actuating means through the hydraulic conduits or lines connected to the cylinder. The operator actuating means includes a suitable source of pressurized hydraulic fluid, including valve means, for selectively delivering hydraulic fluid to one cylinder end and draining hydraulic fluid from the other cylinder end. The fluid source means can be located remotely from the propulsion unit. A steering arm or tiller arm is fixed at one end to the hydraulic cylinder and at the opposite end to the propulsion unit. Thus, steering actuation causes a resultant fluid pressure differential in the cylinder which serves to move the cylinder relative to the support rod, and in turn actuates the steering arm and thereby pivots the propulsion unit.
Power steering systems for marine vehicles mounted remote from the propulsion unit, and overcoming the several disadvantages of the prior art, are disclosed in U.S. Pat. Nos. 5,427,555 and 5,603,279, both of which are assigned to the same assignee as the subject application, and are incorporated by reference into this specification. In a hydraulic power steering system, side-loading forces (e.g., torque) originating from the propulsion unit is overcome, thereby restraining the steering forces. That is, the power steering means reduces the effort at the steering helm or wheel to only the effort required to operate the hydraulic assembly, which is independent of the torque generated by the propulsion unit or any other side-loading forces.
When operating the boat, a variation in side-loading forces (external forces) originating from the propulsion unit or engine will cause movement in the engine due to play in the steering system (sometimes referred to as a "loose" engine), and will cause the boat to rock. Because of this instability, close attention at the steering wheel is required by the driver. The problem can be aggravated by many external forces such as driving the boat in the wake of another, or across a wave, or when changing speed, or upon hitting an object in the water, or the like. As used herein and in the appended claims, the term "external forces" is intended to include all such forces other than the force originated by the operator at the steering helm. With a conventional steering cylinder, such as described above, there is a large disparity between the hydraulic fluid pressures on each side of the piston. For example, torque originating from the engine creates high pressure on one side of the piston, which causes the hydraulic conduits or hydraulic lines connected to that side of the cylinder to expand due to the increase in pressure. This expansion increases the fluid volume in the line on the same side of the cylinder. The fluid volume in the cylinder decreases as the volume in the line increases causing the cylinder to move, and hence the engine is then free to move in that direction. When the torque is decreased, the pressure decreases, the fluid volume from the line returns to the cylinder, which then moves in the opposite direction. As a consequence, the boat is not stable. That is, when torque or other side-loading forces on the engine varies or oscillate, hydraulic pressure on one side or the other of the piston in the steering cylinder changes, i.e., increases or decreases, and causes the boat to be unstable. Any engine movement not initiated by the operator can create control and handling problems such as constant steering correction, and chine walking, and create a generally unstable boat.
This invention has, therefore, as its purpose to provide a steering mechanism that substantially reduces or substantially eliminates control and handling problems resulting from any movement of the engine or rudder not initiated by the operator.